The present invention relates to a color signal processing circuit for separating a modulated color signal and a luminance signal from a composite video signal and demodulating the video signal.
In the hitherto known television receiver of NTSC color television system, separation of the modulated color signal from the composite video signal is generally effected either by using a bandpass filter or through subtraction between a composite video signal and the one delayed therefrom for a single horizontal scanning period (hereinafter represented simply by 1H). Although the separation of the modulated color signal through the bandpass filter is most familiar at present, it suffers from a so-called cross-color phenomenon in which high frequency components of the luminance signals are undesirably mixed in the modulated color signal, as a result of which an image region which inherently consists of fine stripes in white and black, for example, is stained to give a remarkably unnatural appearance. In the latter case, a comb filter is employed which is immune to the cross-color phenomenon. However, when the delay lines constituted by LC-elements or ultrasonic delay lines are used for the comb filter, there arises a problem of instability due to temperature change. The use of a single charge transfer device (hereinafter referred to also as CTD) on the other hand requires disadvantageously a high frequency clock signal as well as an increased number of CTD stages. As an attempt to avoid these drawbacks, there has been proposed a comb filter circuit in which two CTD's are used with the subcarrier signal being utilized as the clock signal. A typical example of a color demodulator circuit which includes a pair of such comb filters in combination is shown in FIG. 1 in a block diagram in FIG. 1 of the accompanying drawings. In FIG. 1, reference numeral 1 denotes a bandpass filter. A broken line block 2 indicates a first comb filter circuit which comprises a first CTD 3, a second CTD 4 providing a delay time longer by 1H than that of the first CTD, and a first subtractor 5. Numeral 6 denotes a first low-pass filter. A broken line block 7 represents a second comb filter circuit which is constituted by a third CTD, a fourth CTD providing a delay time longer by 1H than that of the third CTD and a second subtractor 10. Numeral 11 denotes a second low-pass filter. The color demodulator circuit further includes a color synchronous circuit 12, a first frequency multiplier 13, a first clock pulse generator 14, a phase shifter 15, a second frequency multiplier 16 and a second clock pulse generator 17. FIG. 2 shows waveforms of signal at various circuit points in the color demodulator circuit shown in FIG. 1 to illustrate the operation thereof. Upon separation of modulated color signals from the composite video signal at the bandpass filter 1, high frequency components of the luminance signal in the frequency band of the modulated color signal will be simultaneously eliminated. A continuous subcarrier signal having a phase difference of 90.degree. relative to the phase of the color burst signal is reproduced at the color synchronous circuit 12 from the color burst signal in the modulated color signal which is separated from the composite video signal. The frequency of the subcarrier signal is multiplied by a factor of 2 at the first frequency multiplier 13 and then divided by the clock pulse generator 14 thereby to produce clock pulse signals having a frequency of the subcarrier signal and being out-of-phase from each other by 180.degree., as shown at (a) and (b) in FIG. 2. These clock pulse signals are utilized for driving the first and second CTD's 3 and 4. Through the first CTD 3, input signal is sampled at the time point of the falling or trailing edge of the clock pulse signal (a). Since the input signal to the first CTD 3 is such a modulated color signal as shown at (c) in FIG. 2, the output from the first CTD 3 will be such a sampled color signal as shown at (d) in FIG. 2, provided that the time delay at the first CTD 3 is neglected. Further, because of longer delay time of the second CTD by 1H than the first CTD 3, the output signal from the second CTD 4 is delayed for the duration of 1H relative to the output signal from the first CTD 3. Since the modulated color signal has an interleave frequency relative to that of the luminance signal, the preceding signal in advance of 1H will be such as shown by the waveform (e) in FIG. 2. The second CTD 4 is driven by a clock signal of the opposite phase to the clock signal for driving the first CTD 3, and the input signal to the second CTD 4 is sampled at the time point of the leading edge of the clock pulse shown at (b) in FIG. 2. Since the clock pulse frequency is equal to that of the subcarrier signal interleaving the luminance signal frequency, the sampling at the second CTD 4 is effected at a time point delayed by 1H relative to the time point of the sampling at the first CTD 3. Thus, the output signal from the second CTD 4 will be such a sampled signal as shown at (f) in FIG. 2.
A signal (g) can be obtained by subtracting the output signal of the second CTD 4 from that of the first CTD 3. Application of the signal (g) to the low-pass filter 6 will result in the output signal (h) shown in FIG. 2, which corresponds to a demodulated signal of the modulated signal (c) in FIG. 2 at the trailing edge (phase) of the clock pulse (a). Accordingly, when the trailing phase of the clock pulse (a) shown in FIG. 2 is set so as to be equal to the axis (R-Y), then the demodulated (R-Y) signal can be obtained at the output of the low-pass filter 6.
The luminance signal in the frequency band of the modulated color signal is concentrated on a frequency equal to an integral multiplication of the horizontal repetition frequency. Accordingly, if the signal (c) of FIG. 2 is the luminance signal, the signal (c) before undergoing the delay of 1H will be of the same waveform as the signal (e), as is shown at (i) in FIG. 2, which means that the output signal from the second CTD 4 shown at (j) in FIG. 2 is substantially same as the waveform (d). Thus, subtraction of the output of the second CTD 4 from the output of the first CTD 3 at the subtractor 5 will result in approximately zero output. In this manner, it is possible to obtain the demodulated (R-Y) signal having no luminance signal components from the modulated color signal mixed with the luminance signal. By phase-shifting the subcarrier signal to the axis (B-Y) through the phase shifter 15, it is possible to obtain the demodulated (B-Y) signal having no luminance signal components through operation of the comb filter 7 in the similar manner as the first comb filter 2 described above.
With the circuit arrangement shown in FIG. 1, it is thus possible to separate the luminance signal component from the color signal component. However, this circuit is inoperative for the elimination of the modulated color signal from the luminance signal. In other words, the above described comb filter circuit is intended to be inserted in the color signal channel and can not be applied to the luminance signal channel. Besides, since the output signal from the comb filter circuit is the sampled color signal which includes a demodulated color signal and some harmonics thereof it is impossible to eliminate the modulated color signal superposed on the luminance signal with the aid of the output signal from the comb filter circuit. Accordingly, a so-called dot interference which occurs due to the fact that the color signal is inputted to the color CRT through the luminance signal channel, can not be obviated. In this respect, it can be said that the operation of the comb filter circuit is unsatisfactory as compared with the prior known filter circuit employing the delay lines of RC-elements.